1. Technical Field
The present invention relates to a doped fiber amplifier arrangement and, more particularly, to a amplifier arrangement requiring only a single multiplexer to provide communication and pump signal inputs to multiple amplifier stages.
2. Description of the Prior Art
In many lightwave communication systems there is an increasing need to provide direct optical amplification, in contrast to prior arrangements which utilized opto-electronic converters and electronic regenerators/repeaters to boost the signal power. Some applications include undersea voice and data communication systems, and terrestrial broadband distribution systems, such as CATV, fiber in the loop (FITL) or fiber to the home (FTTH). There is a considerable effort under way to develop rare earth doped fiber amplifiers to fill these needs. The rare earth doped optical amplifying fibers are know to have low cost, low noise properties, a relatively large bandwidth (which is not polarization dependent), minimal crosstalk problems, and relatively low insertion losses at the wavelengths which are used in these systems (for example, .lambda..sub.signal =1500 nm). In use, rare earth doped optical fiber amplifiers are usually coupled end-to-end with an optical communication fiber and are further coupled (using a directional coupler) to a laser diode pump signal source. The presence of the pump signal (at a particular wavelength, for example, 980 nm or 1480 nm) with the communication signal within the rare earth doped fiber results in optical gain of the communication signal. Advantageously, the gain in independent of the propagation direction of the pump signal. Therefore, doped fiber amplifier arrangements may utilize pump signals propagating in either direction and are often described as utilizing "co-propagating pumps" or "counter-propagating pumps".
In most applications, multiple pump sources are utilized so that the loss of a single pump does not result in loss of amplification. See, for example, U.S. Pat. No. 5,050,949 issued to D. J. DiGiovanni et al. on Sep. 24, 1991 entitled "Multi-Stage Optical Fiber Amplifier", or U.S. Pat. No. 5,058,974 issued to L. F. Mollenauer on Oct. 22, 1991 entitled "Distributed Amplification for Lightwave Transmission System". A problem with these and other prior art arrangements is the need to provide a separate wavelength division multiplexer with each pump source. Conventional co- and counter-propagating amplifier designs use three out of four ports of a multiplexer device (bulk or fiber multiplexers, or integrated optic versions). A first input port is used to couple the pump signal, a second input port to couple the communication signal, and a first output port as a common port to carry both input signals to the amplifying medium (e.g., rare earth doped fiber). Generally, the second output port is blocked or used as a tap for the signal or pump. The use of a separate multiplexer with each pump necessarily increases overall optical loss, fabrication complexity and costs. Further, crosstalk between pumps may lead to gain and noise instability and, ultimately, catastrophic failure of the pumps. Insertion of additional isolators to protect the pumps from optical feedback is a solution to the crosstalk problem, but again increases the size, cost and complexity of the total amplifier.
Therefore, a need remains in the art for an optical fiber amplifier arrangement which provides the advantages of multiple pump sources, without unduly increasing the resulting cost or complexity of the fiber amplifier configuration.